The problem is known for many years, but it seems nobody cares about it. Not only scaling is affected, but almost all transformations of image, because this bug is not related particulary to scaling, but rather to averaging pixels (mostly antialiasing).

even simple Gaussian blur is affected by this bug

Test picture:

Blured - as it should be done:

Blured - by GIMP v2.6.8:

how to achieve such a beautiful blur?

I guess you like the first blur example (2nd picture from the top). Have I a special kind of software to get such effect? No, it is the same buggy GIMP 2.6, but with a little help from me. GIMP processes image with gamma 2.2, but does bluring computations as if it has gamma 1.0, so we must flatten picture's gamma before any transformation and restore it after the job.

Step 1: Apply Gamma modifier of 0.45 (real picture will go darker, but not this one)

Step 2: Apply Gaussian Blur (looks terrible)

Step 3: Apply Gamma modifier of 2.2 (that is it!)

Of course this method is not perfectly accurate and can introduce rounding errors, but the result in most cases will be much better than without this trick.